ble of being traced to any particular part. Of this description are the feelings expressed by the terms uneasiness, weariness, weakness, sickness, and those of an opposite character, as ease, hilarity, health, vigour, &c.

75. Relation between the sensation and what is outwardly signified.

We here return a moment to the subject of the relation between the internal sensation and the outward object, and again repeat that the mental state and the corresponding outward object are altogether diverse. This view holds good in the case of the secondary, as well as of the primary qualities of matter. Whether we speak of extension, or resistance, or heat, or colour, or roughness, there are, in all cases alike, two things, the internal affection and the outward quality; but they are utterly distinct, totally without likeness to each other. But how it happens that one thing, which is totally different from another, can nevertheless give us a knowledge of that from which it differs, it would be a waste of time to attempt to explain. Our knowledge is undoubtedly limited to the mere fact.

This is one of those difficult but decisive points in MENTAL PHILOSOPHY, of which it is essential to possess a precise and correct understanding. The letters which cover over the pages of a book are a very different thing from the thought, and the combinations of thought, which they stand for. The accountant's columns of numerals are not identical with the quantities and their relations which they represent. And so in regard to the mind; all its acts are of one kind, and what they stand for is of another. The mind, in all its feelings and operations, is governed by its own laws, and characterizes its efforts by the essential elements of its own nature. Nothing which is seen or heard, nothing which is the subject of taste, or touch, or any other sense, nothing material which can be imagined to exist in any place or in any form, can furnish the least positive disclosure either of its intrinsic nature or of the mode of its action.

What, then, is the relation between the sensation and the outward object, between the perception and the thing perceived? Evidently that of the sign and the thing sig

nified. And as in a multitude of cases, the sign may give a knowledge of its object, without any other grounds of such knowledge than mere institution or appointment, so it is in this. The mind, maintaining its appropriate action, and utterly rejecting the intervention of all images and visible representations, except what are outward and material, and totally distinct from itself both in place and nature, is, notwithstanding, susceptible of the knowledge of things exterior, and can form an acquaintance with the universe of matter.

A misapprehension in this respect, the mistaken supposition of the mind's either receiving actual filmy images from external objects, or being itself transformed into the likeness of such images, has been, in times past, the source of much confusion and contention. But that opinion, however prevalent it may have been once, is mere hypothesis; it has not the slightest well-founded evidence in its favour. Still we can reject it wholly from our belief, and from all influence on our belief, only by guarding against early associations, by a rigid self-inspection, and by carefully separating the material and the immaterial, the qualities of mind and of matter.

CHAPTER VI.

THE SENSE OF SIGHT.

76. Of the organ of sight, and the uses or benefits of that sense. Of those instruments of external perception with which a benevolent Providence has favoured us, a high rank must be given to the sense of seeing. If we were restricted in the process of acquiring knowledge to the informations of the touch merely, how many embarrassments would attend our progress, and how slow it would prove! Having never possessed sight, it would be many years before the most acute and active person could form an idea of a mountain, or even of a large edifice. But by the additional help of the sense of seeing, he not only observes

the figure of large buildings, but is in a moment possessed of all the beauties of a wide and variegated landscape.

The organ of this sense is the eye. On a slight examination, the eye is found to be a sort of telescope, having its distinct parts, and discovering throughout the most exquisite construction. The medium on which this organ acts are rays of light, everywhere diffused, and always advancing, if they meet with no opposition, in direct lines. The eye, like all the other senses, not only receives externally the medium on which it acts, but carries the rays of light into itself; and, on principles purely scientific, refracts and combines them anew.

It does not, however, fall within our plan to give a minute description of the eye, which belongs rather to the physiologist; but such a description, with the statement of the uses of the different parts of the organ, must be to a candid and reflecting mind a most powerful argument in proof of the existence and goodness of the Supreme Being. How wonderful, among other things, is the adaptation of the rays of light to the eye! If these rays were not of a texture extremely small, they would cause much pain to the organ of vision, into which they so rapidly pass. If they were not capable of exciting within us the sensations of colour, we should be deprived of much of that high satisfaction which we now take in beholding surrounding objects; showing forth, wherever they are to be found, the greatest variety and the utmost richness of tints

77. Statement of the mode or process in visual perception.

In the process of vision, the rays of light, coming from various objects and in various directions, strike, in the first place, on the pellucid or transparent part of the ball of the eye.

If they were to continue passing on precisely in the same direction, they would produce merely one mingled and indistinct expanse of colour. In their progress, however, through the crystalline humour, they are refracted or bent from their former direction, and are distributed to certain focal points on the retina, which is a white, fibrous expansion of the optic nerve.

The rays of light, coming from objects in the field of

vision, whether it be more or less extensive, as soon as they have been distributed on their distinct portions of the retina, and have formed an image there, are immediately followed by the sensation or perception which is termed sight. The image which is thus pictured on the retina, is the last step which we are able to designate in the material part of the process in visual perception; the mental tate follows, but it is not in our power to trace, even in the smallest degree, any physical connexion between the optical image and the corresponding state of the mind.— All that we can say in this case is, that we suppose them to hold to each other the relation of antecedent and consequent by an ultimate law of our constitution.

77. Of the original and acquired perceptions of sight.

In speaking of those sensations and perceptions, the origin of which is generally attributed to the sense of sight, it is necessary to make a distinction between those which are ORIGINAL and those which are ACQUIRED. Nothing is properly original with the sense of sight but the sensations of colour, such as red, blue, yellow. These sensations (or perceptions, as they are otherwise called, when the internal feeling is combined with a reference to the external cause) are exceedingly numerous. In this respect, the intimations of the sense of sight stand on the same footing with those of taste and hearing; although distinctive names, in consequence of the difficulty of accurately separating and drawing the line between each, are given only in a few cases. All the sensations of colour are original with the sight; and are not to be ascribed to any other sense.

A part, however, of that knowledge, which we attribute to the sight, and which has the appearance of being immediate and original in that sense, is not so. Some of its alleged perceptions are properly the results of sensations, combined not only with the usual reference to an external cause, but with various other acts of the judgment. In some cases the combination of the acts of the judgment with the visual sensation is carried so far, that there is a sort of transfer to the sight of the knowledge which has been obtained from some other source. And

not unfrequently, in consequence of a long and tenacious association, we are apt to look upon the knowledge thus acquired as truly original in the seeing power. This will suffice, perhaps, as a statement of the general fact, while the brief examination of a few instances will help to the more thorough understanding of those acquired percep tions of the sight which are here referred to

79. The idea of extension not originally from sight.

It is well known that there is nothing more common than for a person to say that he sees the length or breadth of any external object; that he sees its extent, &c. These expressions appear to imply (and undoubtedly are so understood) that extension is a direct object of sight. There is no question that such is the common sentiment, viz., that the outlines and surface which bodies permanently expand and present to the view, are truly seen. An opinion different from this might even incur the charge of great absurdity.

But, properly, the notion of extension, as we have already seen, has its origin in the sense of touch. Being a simple and elementary thought, it is not susceptible of definition; nor, when we consider extension as existing outwardly and materially, can we make it a matter of description without running into the confusion of using synonymous words. But, whatever it is (and certainly there can be neither ignorance nor disagreement on that point, however much language may fail of conveying our ideas), the knowledge of it is not to be ascribed originally to the sight.

The notion of extension is closely connected with externality. It is not possible to form the idea of extension from mere consciousness, or a reflection on what takes place within us. But making a muscular effort, and thus applying the touch to some resisting body, we first have the notion of outness; and either from the same application of that sense, or when we have repeated it continuously on the same surface, we have the additional notion of its being extended or spread out. If a man were fixed immoveably in one place, capable of smelling, tasting, hearing, and seeing, but without tactual impressions ori